Patients with congestive heart failure (“CHF”) are often treated by surgical implantation of a cardiac stimulation device (such as an implantable cardioverter-defibrillator, or “ICD”) providing one or more stimulation therapies, such as pacing, cardioversion or defibrillation. Such patients generally experience a progressive, increasingly deteriorating, cardiac disease state. Over time, the substrate and function of the heart for a CHF patient changes, notably for the worse.
The changes in a patient's cardiac characteristics over time are likely to affect detrimentally the operation of an implantable cardiac stimulation device. In particular, the defibrillation threshold (“DFT”) parameter determined by the physician at the time of device implant changes over time, typically in the direction of an increasing defibrillation threshold. A concern of an attending physician is whether he or she has programmed a sufficient safety margin into the implanted device to compensate for a possibly dramatic upward shift in the patient's DFT. Many patients are brought back to the hospital within a 3 to 6 month period following implant to determine whether there has been a significant shift upwards and whether the patient's DFT requires adjustment accordingly. A physician may be faced with the prospect of replacing the existing implanted device with one that provides a higher defibrillation energy to meet the higher energy requirement (comprising both the DFT value and the safety margin). The patient is continually placed at risk, and there are significant additional costs incurred as part of a regular follow-up procedure for managing the care of significantly ill CHF patients.
Some attempts have been made at predicting patients' defibrillation threshold changes overtime, but such attempts have been largely confined to the clinical setting. For example, several studies have evaluated clinical variables for the prediction of defibrillation thresholds, using transvenous leads to deliver biphasic defibrillating shocks. More than 40 clinical variables have been assessed, including demographic, electrocardiographic, echocardiographic, and radiographic measurements. However, these clinical parameters are not intrinsic to the cardiac stimulation field.
It would therefore be advantageous to provide an implantable cardiac stimulation device capable of automatically determining a patient's defibrillation energy threshold and related defibrillation parameters, taking account of the changing nature of a patient's cardiac characteristics over time.